Legionnaires’ disease has been associated with domestic hot-water systems in a number of outbreaks.
Large water heaters like those used in hospital or industrial settings frequently contain cool zones near the base where cold water enters and scale and sediment accumulate.
Properly maintaining a hot-water system includes being aware of the following:
Conditions that promote growth:
- Scale and sediment supply the environment needed for growth of Legionnaires’ disease bacteria (LDB) and other microorganisms.
- Dead legs and non-recirculated plumbing lines that allow hot water to stagnate also provide areas for growth of the organism.
- Temperatures maintained below 60°C (140°F) encourage growth of LDB and other microorganisms.
- Maintain domestic water heaters at 60°C (140°F) and water delivered at the faucet at a minimum of 50°C (122°F). Where these temperatures cannot be maintained, control LDB growth with a safe and effective alternative method. Also see What to consider in the system design.
- Proper insulation of hot-water lines and heat tracing of lines can help maintain distribution and delivery temperatures at 50°C (122°F).
- If potential for scalding exists, employ appropriate fail-safe scald-protection.
- Run recirculation pumps continuously and exclude them from energy conservation measures.
- Eliminate or minimize the use of rubber, plastic and silicone gaskets in the plumbing system. These materials may serve as growth substrates for LDB. Frequent flushing of these lines also reduces growth.
- Identify and test the integrity of all backflow preventers (to ensure protection of domestic water from cross-contamination with process water) through a building code-approved method.
- Hot-water tanks should be drained periodically to remove scale and sediment.
- Periodically chlorinate the system at the tank to produce 10 ppm free residual chlorine and flush all taps until a distinct odor of chlorine is evident as a means of control. The tank should be thoroughly rinsed to remove excess chlorine before reuse.
- In-line chlorinators can be installed in the hot water line; however, chlorine is quite corrosive and will shorten the service life of metal plumbing.
- Control of the pH in the range of 6.8 – 7.0 is extremely important to ensure that there is adequate residual chlorine in the system.
Collect water samples to determine potential contamination as outlined above.
Please Note: It is important not to flush a hot-water outlet before taking a sample because the end section of the water system may be a source of contamination.
- Sample the incoming water supply if the plumbing provides access.
- Sample hot and tepid water tanks and reservoirs.
- Sample faucets and showerheads throughout the facility, for example, those nearest, intermediate, and most distant from water heaters, storage tanks, and connections with municipal water supplies.
- If a biocide is used, follow the manufacturer’s instructions for proper neutralization.
- The OSHA suggested guideline for LDB in domestic hot- and tepid-water systems is less than 10 CFU per milliliter.
- If LDB concentrations are below 10 CFU per milliliter and no LDB were detected in swab or other samples, no further monitoring for LDB is necessary. Continue the maintenance program as long as the system is in use.
- If water concentrations exceed 10 CFU per milliliter or LDB were detected in other samples, take steps to identify the source of contamination or amplification and treat the system.
See How to treat a contaminated water system.
- Sample the water system monthly until the source of contamination is identified and adequately treated. Once LDB concentrations remain below 10 CFU for a tree-month period, sampling may be stopped.
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